I see that a lot of people are/were having the same problems as myself with this game (i had previously tried EVERYTHING!!) BUT, yesterday i decided to remove my 32 bit windows 7 and install the 64 bit version, i then installed the latest ATI drivers from here http://downloads.guru3d.com/download.php?det=2801#download, then re-installed the game and now it works perfectly, no texture pop ins, no lag...ultra smooth

Intel core 2 duo 6400 @ 2.13gz
ATI Radeon HD5570 1gb
4Gb Ram
64 bit windows 7 ultimate
250GB hard drive @7200rpm
I haven't entered anything in the steam launch options box or modified the game in any way, the only thing i did do was alter the V-Sync settings to smart in the game menu (No changes to the aa etc)..............Happy Days
